1. Field of the Invention
The present invention is directed to an apparatus and method for evaluating an object, particularly an object larger than an aperture of a sensor.
2. Description of Related Art
The use of wavefront sensors, including Shack-Hartmann wavefront sensors, is a known technique for measuring the wavefront of light. The features of a surface, such as a wafer, an optic, etc., may be measured by reflecting light from the surface and directing it to the wavefront sensor. Wavefront sensors determine wavefront error through slope measurement.
In a Shack-Hartmann test, a plurality of lenslets arranged in an array are used to sample the wavefront. Each lenslet creates a corresponding sub-aperture. The resulting array of spots, which may be interpreted as a physical realization of an optical ray trace, are focused onto a detector. The position of a given focal spot is dependent upon the average wavefront slope over the sub-aperture. The direction of propagation, or wavefront slope, of each of the samples is determined by estimating the focal spot position shift for each lenslet. The wavefront may then be reconstructed from the detected image in a number of known manners. The resolution and sensitivity of the sensor are determined by the lenslet array.
There are several applications of the Shack-Hartmann wavefront sensor. Several of these applications have been extensively developed, with specific devices developed for adaptive optics, measurement of pulsed lasers and laser beam quality, ocular adaptive optics and measurement, and a wide variety of metrology applications. For some applications, the Shack-Hartmann sensor is advantageously applied, since it is relatively insensitive to vibration, independent of source light wavelength, and can be arranged in a simple, compact and robust assembly. A summary of uses of Shack-Hartmann wavefront sensors is set forth in D. R. Neal et al. "Wavefront Sensors for Control and Process Monitoring in Optics Manufacture," Lasers as Tools for Manufacturing II, SPIE Volume 2993 (1997).
However, there are a number of metrology applications where the size of the target is a limiting factor in the application of wavefront or other metrology technology. Examples include large mirrors or optics, commercial glass, flat-panel displays and silicon wafers. While some previous methods have been developed, e.g., U.S. Pat. No. 5,563,709 to Poultney, which is hereby incorporated by reference in its entirety for all purposes, these suffer from a loss of spatial resolution when applied to large elements; and from difficulties in size and calibration.
An example of such a metrology application is the measurement of a silicon wafer. In such a measurement, the key result is the determination of surface defects that affect the fabrication of small features on the silicon wafer. The minimum feature size for microelectronic circuits has steadily decreased since their inception. Where 0.35 .mu.m features are currently the norm, the next generation of circuits will need 0.18 .mu.m or even 0.13 .mu.m. Fabrication of these small features requires the detection (and elimination) of ever smaller size defects. At the same time, the wafer size is getting larger. The current generation of 200 mm wafers is rapidly being supplanted by the 300 mm wafer, with 450 mm wafers planned for the near future. The need for ever better resolution, combined with larger wafers places extremely difficult demands upon the metrology tools.
The current generation of metrology methods is clearly not scalable to the needs of these new processes. Such scaling to larger sizes requires extremely large optics with their associated high cost, large footprint and difficulty of fabrication. Furthermore, the required resolution cannot reasonably be obtained with such methods. The Shack-Hartmann method requires at least four pixels per lenslet. Thus, the resolution over a given aperture is limited. Scaling to larger areas with methods such as disclosed in Poultney, requires the use of cameras with an extremely large number of pixels. While the interferometry methods may be applied to larger areas with less loss in resolution, modern practical methods required the acquisition of 4-6 frames of data. This leads to difficulties in automated inspection in a clean-room environment because of vibration and to throughput reduction when analyzing a large object.
Other applications may be even more stressing than the wafer analysis discussed above. While silicon wafers may be scaled to 300 mm or even 450 mm, flat panel displays are currently being fabricated at 1500.times.600 mm. Scaling of existing metrology tools for single aperture measurement is clearly impractical. Automotive or commercial glass is manufactured in even larger areas, with 4 m wide segments not uncommon. Clearly an alternative technique is needed.
As the feature size to be analyzed decreases, the size of tolerable distortions decreases, and high resolution measurements must be made to insure sufficient surface flatness. This high resolution requirement is incompatible with making measurements over a large area. Further, the calibration of a system for measuring flatness over a large area in a single measurement requires a reference of similar dimensions, which is difficult to produce.
While some solutions, such as those set forth in U.S. Pat. No. 4,689,491 to Lindow et al., U.S. Pat. No. 4,730,927 to Ototake et al. and U.S. Pat. No. 5,293,216 to Moslehi disclose point by point analysis of surfaces, the analyzing disclosed in these patents is very time consuming.